Rep. William Lehman (D-Fla.), 71, a soft-spoken former used car salesman with a permanent Florida tan, would never win a part in a spy thriller.

But in October, he found himself speeding through the streets of Tbilisi, capital of Soviet Georgia, hoping that his cab driver was not a KGB agent who would interfere with his mission.

Lehman, making his first trip to the Soviet Union, thought the whole idea of international intrigue was "sort of ridiculous." But he played the part to the hilt.

He said he "simply couldn't take a chance" that the small box he had smuggled into the country in his briefcase would be confiscated.

It contained a $2,000 synthetic heart valve that relatives in the United States of an ailing 22-year-old Soviet woman had requested he deliver.

"I didn't want to do anything to endanger the young woman's life," said the North Miami Beach member of Congress.

Lehman and an aide, Adele Liskov, slipped away from their hotel to a streetside phone booth one afternoon. They gave a contact number to a nervous State Department interpreter with them and asked him to dial it.

They were given an address and a cryptic message: "Look for a woman in red standing next to a short man."

The interpreter left them on their own in a taxi. Neither Lehman nor Liskov spoke the language. They were unfamiliar with the city, although the representative said he felt strangely at home. "It reminded me of Atlanta. Once you've seen one Georgia you've seen them all," he said.

The address, however, was incorrect. Lehman decided to trust the cab driver, an Armenian, because "he drove like a cowboy."

"The way this guy drove, nobody could have followed us," Liskov added.

Luckily, the driver knew the family, and he took the pair to their apartment. The lady in red, the short man and a host of family members were waiting with a celebration. They showered Lehman with gifts of fruit and Armenian brandy.

Lehman said he never had any second thoughts about undertaking the mission.

He did not tell Florida reporters until last week when he was certain that the ailing woman had recovered. But now Lehman wonders if he should have kept it quiet.

The family here that asked him to deliver the heart valve is worried about their relatives and yesterday refused to be interviewed. The State Department refused comment. And it is unclear whether Lehman's negotiations with Soviet officials over the fate of Soviet dissidents will be hindered. The mission began late last summer when Hachig Seyranian, a Soviet emigre from Niagara Falls, N.Y., received a phone call from a brother still living in the Soviet Union.

He was told that his niece, Vartitar, needed a heart operation but was unable to get an artificial heart valve in the Soviet Union.

Using a prescription written by a sympathetic doctor, Seyranian bought a heart valve in New York. But then he could not find anyone to deliver it.

The Soviet Union bans the import of many technical and medical devices. Persons familiar with the Soviet mail system said all packages arriving in the country are opened and their contents often intercepted.

Several friends returning to the Soviet Union for visits refused to take the valve because they feared it would cause them problems at the border. "People were afraid to take it," Seyranian told The Miami Herald last week. "When you leave the free world, things are different." Finally, the State Department told Seyranian of Lehman's planned visit.

Lehman agreed to the mission as soon as he heard about the ailing young woman. His daughter, Kathryn, died five years ago of a brain tumor.

"She was only 22 and that gave me an extra motive," Lehman said. "I kept thinking, 'What would Kathryn want me to do?' "

Lehman said he had no trouble getting the heart valve into the Soviet Union. He simply slipped it into his briefcase, which was never searched, and walked in.

"I didn't treat it like contraband. I treated it like cough drops or something else I was carrying," Lehman said.

Lehman already had undertaken one seemingly hopeless mission, securing release of a young Argentine woman, who had been jailed without charge following the terror that gripped Argentina when President Isabel Peron was deposed.

He said the woman, Esther Debora Benchoam, now a student at Georgetown University, became his "surrogate daughter."